Various types of compound semiconductor devices, each including a compound semiconductor such as SiC and GaN, have been proposed. Among these compound semiconductor devices, there is a type provided with a vertical switching element such as a vertical metal-oxide-semiconductor field-effect transistor (MOSFET) that has an inverted type trench gate structure.
The vertical MOSFET having an inverted type trench gate structure causes such an action that applies a gate voltage to a gate electrode inside a trench to form a channel in a p-type base area located on a trench side face, and produces an electrical current flowing between a drain and a source via the channel thus formed. According to this type of vertical MOSFET, on-resistance and element breakdown voltage are largely dependent on an impurity concentration of the p-type base area. More specifically, channel mobility increases as the impurity concentration of the p-type base area decreases. In this case, channel resistance lowers, wherefore reduction of on-resistance is achievable. However, a breakdown voltage between the drain and the source drops by a punch-through phenomenon caused in this state. As a result, the element breakdown voltage drops. Accordingly, achievement of both high channel mobility and a sufficient element breakdown voltage is essentially difficult.
To address this, control is required in such a manner as to change the impurity concentration of the p-type base area, which includes the channel, as sharply as possible, for example. More specifically, the p-type base area is formed on an n−-type drift layer through epitaxial growth. When the impurity concentration of the p-type base area slowly increases, a thickness of the p-type base area sufficient for obtaining desired characteristics, i.e., a channel length, increases. Accordingly, it is required to change the impurity concentration of the p-type base area as sharply as possible.
Examples of a method for sharply changing an impurity concentration of an impurity layer as described above include a method for sharply changing an impurity concentration of an n-type impurity layer as disclosed in Patent Literature 1. According to this method, propane (C3H8) and silane (SiH4) constituting a material gas for SiC are introduced to develop an undoped layer. In an intermediate period of the introduction, the introduction quantity of propane starts to be reduced, while nitrogen (N2) constituting n-type dopant starts to be introduced. In this manner, the introduction quantity of propane constituting the material gas is reduced during introduction of n-type dopant to change a C/Si ratio. Accordingly, a change of the impurity concentration of the n-type impurity layer is achievable.